Add rust-project.json generator

Because we are not relying on Cargo.toml for our crate dependencies,
we need to provide a structured file to rust-analyzer which describes
the locations of the crates. Add a generator for that purpose, similarly
to cc/compdb and cc/ccdeps.

Bug: 156395307
Test: SOONG_GEN_RUST_PROJECT=1 m nothing && \
      cat ${ANDROID_BUILD_TOP}/out/soong/rust-project.json
Change-Id: I46efe0adeddae281eaf86707504c3aa15b5e80b8